E.g. you connect to Listener on serverA using port 1521, requesting a db connection to service1. The Listener does some form of load balancing, and decides that serverC is best to service service1, and sends a redirect you to connect to serverC on port 1527. Your platform cannot resolve the serverC hostname. Or access to that IP address fails (routing or firewalled). Or port 1527 is by default closed by the firewall.

To troubleshoot:

Enable SQL*Net tracing (via sqlnet,ora) on the client.

Or, list services on Listener you are connecting to and determine just where else (other Listeners, RAC typically) that service is also supported. Build a definitive list of IP addresses and hostnames from this. On your client platform, ensure hostname resolution works, IPs are accessible, and ports are not firewalled.

Your question would have been better suited to the General Database Discussions forum, as it's nothing to do with SQL or PL/SQL. Please ensure you post in the correct forum in future, as this also means that you'll target the correct audience and likely receive better support (not that the support you've already got here isn't good hehe!)